To set up a connection in this kind of network, it is necessary to determine not only a spatial route, consisting of a sequence of route segments connecting the starting node to the destination node, but also a spectral route, since each segment is able to support a plurality of wavelengths each constituting a spectral route segment. Selecting a spectral route entails selecting the wavelength to be used, or the wavelengths to be used successively, on different segments along the spatial route. It is sometimes necessary to carry out operations on the signal and/or the information transported, necessitating the addition of dedicated equipment to the network and the use thereof. However, such processing operations are costly, and it is therefore desirable to avoid them as much as possible. They may relate to regeneration and/or wavelength conversion, which may be carried out by purely optical means or using optical-electrical and electrical-optical conversion means.
The expression “wavelength continuity” refers to the use of the same wavelength from the starting node to the destination node, even if operations on the signal and/or the information transported require optical-electronic-optical conversion or 1R, 2R, or 3R regeneration.
The term “transparency” is used, a distinction being drawn between different types of transparency, depending on whether optical-electronic-optical conversion, wavelength conversion, 1R, 2R or 3R regeneration, or a combination of these operations are avoided. The aim is to avoid the route for the signal passing through “non-transparency points” or, if this is not possible, to minimize the number of non-transparency points the signal passes through. For example, in the case of a form of transparency consisting in the absence of optical-electronic-optical conversion, the aim is to minimize the number of times the optical signal passes through opto-electronic and electronic-optical interfaces. If it is not possible to avoid conversion completely, then a route is looked for that minimizes the number of conversions needed.
There may additionally be connection capacity constraints or quality-of-service constraints that influence the selection of the spatial route and the spectral route.
At present, there is no satisfactory method of determining a transparent route of the above kind. One method was proposed to the IETF in Generalized MPLS—Signaling Functional Description, chapters 3.4 and 3.5, Expiration date: November 2001, Network Working Group, Internet Draft.
That prior art method consists in:                using a conventional routing method to determine a spatial route connecting a starting node to a destination node and comprising a sequence of route segments, each segment interconnecting two nodes of the network directly;        determining a first set of wavelengths, in the starting node, for communicating with the next node constituting the route, i.e. the second node on the route;        determining, from the set of wavelengths proposed by the preceding node, a second set of wavelengths, in the second node, for communicating with the next node constituting the route, i.e. the third node on the route;        in the nth node, determining, from the set of wavelengths proposed by the preceding node, an (n+1)th set of wavelengths, for communicating with the next node constituting the route, i.e. the (n+1)th node on the route; and so on, as far as the destination node.        
The object of the above prior art method is only to assure wavelength continuity. The nodes do not propagate the sets of wavelengths because they do not need to do this to find a spectral route assuring wavelength continuity (should one exist end to end).
Each node of the route may retain or reduce the set of wavelengths that it inherits from the upstream node, according to the resources available for the connection to the downstream node. A transparent route is finally set up if the resulting set contains at least one wavelength. A drawback of that method is a high probability of blocking, since the selection made locally at each node may reduce options in downstream nodes. That method therefore constitutes a sub-optimum solution, or even no solution at all in some cases, when there is in fact an acceptable solution, although it is not transparent end to end. Also, the above prior art method takes account of only one parameter: the continuity of a given wavelength.